1. Field of the Invention
This invention concerns a process for preparing an aqueous dye concentrate wherein the dye comprises a benzoylated naphthylamine segment coupled to the diazonium salt of p-(p-aminophenylazo)benzene sodium sulfonate.
2. Description of the Prior Art
It is known to form the dye ##STR1## by diazotizing p-(p-aminophenylazo)benzenesulfonic acid, and coupling to N-benzoyl J acid in the presence of a strong alkali. When the dye is dissolved in water the solution changes shade over a period of days. The change in color results from the slow alkaline hydrolysis of the dye, the benzoyl group being removed thereby. When a dye solution is prepared from the reaction mass resulting from the coupling reaction noted above, the problem of change of shade is accentuated, since a greater degree of alkalinity is necessary to overcome the salting out effect of the additional impurities present.
The difficulties of this situation are overcome by the process of the present invention which employs a mildly basic tertiary amine to neutralize HCl released during benzoylation and also as acid acceptor during the coupling reaction. The effectiveness of these tertiary amines in the benzoylation is not taught or suggested by the prior art. While the invention is illustrated with and has found most use with the benzamide derived from J acid, 6-amino-1-naphthol-3-sulfonic acid, it can be applied to other naphthalene amino compounds of comparable reactivity to produce dye concentrates having substantially constant strength of shade.